The present invention relates generally to plasma cutting systems and, more particularly, to a post arc gas flow control for such systems.
Plasma cutting is a process in which an electric arc is used for cutting or gouging a workpiece. Plasma cutters typically include a power source, an air supply, and a torch. The torch, or plasma torch, is used to create and maintain the plasma arc that performs the cutting/gouging operation. A plasma cutting power source typically receives an input voltage from a transmission power receptacle or generator and provides output power to a pair of output terminals, one of which is connected to an electrode and the other of which is connected to the workpiece. An air supply, either internal or external, is used to carry and propel the arc to the workpiece and cool the torch head.
There are multiple ways of initiating the cutting process, such as contact start, high frequency or high voltage starting. Generally, in contact start plasma cutters, a movable or fixed electrode or consumable serves as a cathode and a fixed or movable nozzle or tip serves as an anode. In some units, the air supply is used to force a separation of the electrode and tip to create an initial or pilot arc. In others, mechanical or electromechanical means can serve to separate the contacts and generate the pilot arc. In either case, once the pilot arc is established, air is forced past the pilot arc whereby it is heated and ionized to form a plasma jet that is forced out of the torch through the opening in the nozzle. The air aids in extending the arc to the workpiece forming a cutting arc and initiating the cutting process.
Both the pilot arc and the cutting arc are electrically supported by the electrode of the plasma torch. Considerable heat is generated during the plasma generating process. The plasma torch must be constructed to withstand considerable heat and power concentration associated with the plasma cutting process. After arc termination, the plasma cutting torch must dissipate the residual heat generated during the cutting process. Known plasma cutting systems dissipate this heat by maintaining an air flow through the torch after arc termination for a predefined time duration. That is, after arc termination, air is allowed to continue to flow through the torch for a preset period. The flow of gas through the torch after arc termination is commonly referred to post flow cooling.
Allowing air to flow through the torch for a preset duration is generally inefficient. The amount of heat that must be removed from the torch after arc termination is directly related to several factors: the duration of the cutting arc, the power level required to perform a cutting process, the type of cutting process performed, the type of tip assembly utilized, and the operator. The higher the temperature associated with the plasma cutting process, the more heat that must be removed from the torch after termination of the plasma cutting process.
Maintaining the post flow of cooling gas for a preset duration disregards the actual arc termination temperature of the plasma torch. That is, the preset duration of post arc cooling flow either frequently provides more cooling than is necessary or terminates before adequate cooling has been achieved. The preset cooling duration is indifferent to the type of torch tip assembly utilized, the plasma cutting process duration, the type of plasma process performed, the operational power associated with the plasma process, and/or the way the operator is performing the operation. Premature termination of the post flow cooling can adversely affect the life cycle of the plasma torch tip assembly and post flow cooling beyond adequate cooling of the tip assembly consumes more cooling gas than is required.
Furthermore, when the torch has been adequately cooled prior to termination of the preset cooling duration, the continued flow of cooling gas through the torch requires continued generation of cooling gas after the plasma torch has been adequately cooled. If the cooling gas is supplied from an enclosed source, such as bottled gas, this continued operation results in the premature depletion of the gas source. If the cooling gas is supplied from a compressor, the unnecessary continuation of the cooling flow results in inefficient utilization of the compressor.
It would, therefore, be desirable to design a plasma cutting system that dynamically controls the post arc cooling flow.